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192 lines
7.2 KiB
Markdown
192 lines
7.2 KiB
Markdown
# Metadata
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This draft recaps the current metadata handling in GStreamer and
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proposes some additions.
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## Supported Metadata standards
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The paragraphs below list supported native metadata standards sorted by
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type and then in alphabetical order. Some standards have been extended
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to support additional metadata. GStreamer already supports all of those
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to some extend. This is showns in the table below as either `[--]`,
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`[r-]`, `[-w]` or `[rw]` depending on read/write support (08.Feb.2010).
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### Audio
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- mp3
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* ID3v2: `[rw]`
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* http://www.id3.org/Developer_Information
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* ID3v1: `[rw]`
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* http://www.id3.org/ID3v1
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* XMP: `[--]` (inside ID3v2 PRIV tag of owner XMP)
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* http://www.adobe.com/devnet/xmp/
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- ogg/vorbis
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* vorbiscomment: `[rw]`
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* http://www.xiph.org/vorbis/doc/v-comment.html
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* http://wiki.xiph.org/VorbisComment
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- wav
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* LIST/INFO chunk: `[rw]`
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* http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/RIFF.html#Info
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* http://www.kk.iij4u.or.jp/~kondo/wave/mpidata.txt
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* XMP: `[--]`
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* http://www.adobe.com/devnet/xmp/
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### Video
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- 3gp
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* {moov,trak}.udta: `[rw]`
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* http://www.3gpp.org/ftp/Specs/html-info/26244.htm
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* ID3V2: `[--]`
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* http://www.3gpp.org/ftp/Specs/html-info/26244.htm
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* http://www.mp4ra.org/specs.html#id3v2
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- avi
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* LIST/INFO chunk: `[rw]`
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* http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/RIFF.html#Info
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* http://www.kk.iij4u.or.jp/~kondo/wave/mpidata.txt
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* XMP: `[--]` (inside "_PMX" chunk)
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* http://www.adobe.com/devnet/xmp/
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- asf
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* ??:
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* XMP: `[--]`
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* http://www.adobe.com/devnet/xmp/
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- flv `[--]`
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* XMP: (inside onXMPData script data tag)
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* http://www.adobe.com/devnet/xmp/
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- mkv
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* tags: `[rw]`
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* http://www.matroska.org/technical/specs/tagging/index.html
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- mov
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* XMP: `[--]` (inside moov.udta.XMP_ box)
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* http://www.adobe.com/devnet/xmp/
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- mp4
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* {moov,trak}.udta: `[rw]`
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* http://standards.iso.org/ittf/PubliclyAvailableStandards/c051533_ISO_IEC_14496-12_2008.zip
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* moov.udta.meta.ilst: `[rw]`
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* http://atomicparsley.sourceforge.net/
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* http://atomicparsley.sourceforge.net/mpeg-4files.html
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* ID3v2: `[--]`
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* http://www.mp4ra.org/specs.html#id3v2
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* XMP: `[--]` (inside UUID box)
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* http://www.adobe.com/devnet/xmp/
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- mxf
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* ??
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### Images
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- gif
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* XMP: `[--]`
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* http://www.adobe.com/devnet/xmp/
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- jpg
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* jif: `[rw]` (only comments)
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* EXIF: `[rw]` (via metadata plugin)
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* http://www.exif.org/specifications.html
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* IPTC: `[rw]` (via metadata plugin)
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* http://www.iptc.org/IPTC4XMP/
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* XMP: `[rw]` (via metadata plugin)
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* http://www.adobe.com/devnet/xmp/
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- png
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* XMP: `[--]`
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* http://www.adobe.com/devnet/xmp/
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### further Links:
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http://age.hobba.nl/audio/tag_frame_reference.html
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http://wiki.creativecommons.org/Tracker_CC_Indexing
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## Current Metadata handling
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When reading files, demuxers or parsers extract the metadata. It will be
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sent a `GST_EVENT_TAG` to downstream elements. When a sink element
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receives a tag event, it will post a `GST_MESSAGE_TAG` message on the
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bus with the contents of the tag event.
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Elements receiving `GST_EVENT_TAG` events can mangle them, mux them into
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the buffers they send or just pass them through. Usually is muxers that
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will format the tag data into the form required by the format they mux.
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Such elements would also implement the `GstTagSetter` interface to receive
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tags from the application.
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```
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+----------+
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| demux |
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sink src --> GstEvent(tag) over GstPad to downstream element
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+----------+
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method call over GstTagSetter interface from application
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v
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+----------+
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| mux |
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GstEvent(tag) over GstPad from upstream element --> sink src
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+----------+
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```
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The data used in all those interfaces is `GstTagList`. It is based on a
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`GstStructure` which is like a hash table with differently typed entries.
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The key is always a string/GQuark. Many keys are predefined in GStreamer
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core. More keys are defined in gst-plugins-base/gst-libs/gst/tag/tag.h.
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If elements and applications use predefined types, it is possible to
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transcode a file from one format into another while preserving all known
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and mapped metadata.
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## Issues
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### Unknown/Unmapped metadata
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Right now GStreamer can lose metadata when transcoding and/or remuxing
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content. This can happens as we don’t map all metadata fields to generic
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ones.
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We should probably also add the whole metadata blob to the `GstTagList`.
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We would need a `GST_TAG_SYSTEM_xxx` define (e.g.
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`GST_TAG_SYSTEM_ID3V2`) for each standard. The content is not printable
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and should be treated as binary if not known. The tag is not mergeable -
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call `gst_tag_register()` with `GstTagMergeFunc=NULL`. Also the tag data
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is only useful for upstream elements, not for the application.
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A muxer would first scan a taglist for known system tags. Unknown tags
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are ignored as already. It would first populate its own metadata store
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with the entries from the system tag and the update the entries with the
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data in normal tags.
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Below is an initial list of tag systems: `ID3V1` - `GST_TAG_SYSTEM_ID3V1`
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`ID3V2` - `GST_TAG_SYSTEM_ID3V2` `RIFF_INFO` -
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`GST_TAG_SYSTEM_RIFF_INFO` XMP - `GST_TAG_SYSTEM_XMP`
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We would basically need this for each container format.
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See also <https://bugzilla.gnome.org/show_bug.cgi?id=345352>
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### Lost metadata
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A case slighly different from the previous is that when an application
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sets a `GstTagList` on a pipeline. Right elements consuming tags do not
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report which tags have been consumed. Especially when using elements
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that make metadata persistent, we have no means of knowing which of the
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tags made it into the target stream and which were not serialized.
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Ideally the application would like to know which kind of metadata is
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accepted by a pipleine to reflect that in the UI.
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Although it is in practise so that elements implementing `GstTagSetter`
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are the ones that serialize, this does not have to be so. Otherwise we
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could add a means to that interface, where elements add the tags they
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have serialized. The application could build one list from all the tag
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messages and then query all the serialized tags from tag-setters. The
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delta tells what has not been serialized.
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A different approach would be to query the list of supported tags in
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advance. This could be a query (`GST_QUERY_TAG_SUPPORT`). The query
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result could be a list of elements and their tags. As a convenience we
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could flatten the list of tags for the top-level element (if the query
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was sent to a bin) and add that.
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### Tags are per Element
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In many cases we want tags per stream. Even metadata standards like
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mp4/3gp metadata supports that. Right now `GST_MESSAGE_SRC(tags)` is the
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element. We tried changing that to the pad, but that broke applications.
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Also we miss the symmetric functionality in `GstTagSetter`. This interface
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is usually implemented by
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elements.
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### Open bugs
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<https://bugzilla.gnome.org/buglist.cgi?query_format=advanced;short_desc=tag;bug_status=UNCONFIRMED;bug_status=NEW;bug_status=ASSIGNED;bug_status=REOPENED;bug_status=NEEDINFO;short_desc_type=allwordssubstr;product=GStreamer>
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